The chemtrail conspiracy theory holds that some contrails are actually chemicals or biological agents deliberately sprayed at high altitudes for a purpose undisclosed to the general public. Versions of the chemtrail conspiracy theory circulating on the internet and radio talk shows theorize that the activity is directed by government officials. As a result, federal agencies have received thousands of complaints from people who have demanded an explanation. The existence of chemtrails has been repeatedly denied by government agencies and scientists around the world.

The United States Air Force has stated that the theory is a hoax which "has been investigated and refuted by many established and accredited universities, scientific organizations, and major media publications". The British Department for Environment, Food and Rural Affairs has stated that chemtrails "are not scientifically recognised phenomena". The Canadian Leader of the Government in the House of Commons has stated that "The term 'chemtrails' is a popularized expression, and there is no scientific evidence to support their existence."

The term chemtrail is derived from "chemical trail" in the similar fashion that contrail is an abbreviation for condensation trail. It does not refer to common forms of aerial spraying such as crop dusting, cloud seeding or aerial firefighting. The term specifically refers to aerial trails allegedly caused by the systematic high-altitude release of chemical substances not found in ordinary contrails, resulting in the appearance of supposedly uncharacteristic sky tracks. Believers of this theory speculate that the purpose of the chemical release may be for global dimming, population control, weather control, or biowarfare and claim that these trails are causing respiratory illnesses and other health problems.